How often does Sap of Sosaria spawn?

I want to do the human to elf quest but I can't find the Sap of Sosaria on the ground. According to stratics, it is on the ground near the two Yew tress north of Yew Cemetery. I have searched every Yew Tree in Yew Area and still failed to find one sap. Using the search web site, I do find some vendors selling them at 5000 gp a sap. I want to know how often this quest item spawn. Because I failed to find one last night and this morning after the server backed up. Thanks.

By the way, this is in Chesapeake. I know in some servers it is 100 gp a sap. But some smart guys in Chesapeake who take all the Sap of Sosaria and you can only buy them from his vendor. That is why I asked how often this item spawns so I can beat him next time it spawns.

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